Michelangelo and Giacomo della Porta, 1547 and 1585
Michelangelo redesigned the dome in 1547, taking into account all that had gone before. His dome, like that of Florence, is constructed of two shells of brick, the outer one having 16 stone ribs, twice the number at Florence but far fewer than in Sangallo's design. As with the designs of Bramante and Sangallo, the dome is raised from the piers on a drum. The encircling peristyle of Bramante and the arcade of Sangallo are reduced to 16 pairs of Corinthian columns, each of 15 metres (49 ft) high which stand proud of the building, connected by an arch. Visually they appear to buttress each of the ribs, but structurally they are probably quite redundant. The reason for this is that the dome is ovoid in shape, rising steeply as does the dome of Florence Cathedral, and therefore exerting less outward thrust than does a hemispherical dome, such as that of the Pantheon, which, although it is not buttressed, is countered by the downward thrust of heavy masonry which extends above the circling wall. The ovoid profile of the dome has been the subject of much speculation and scholarship over the past century. Michelangelo died in 1564, leaving the drum of the dome complete, and Bramante's piers much bulkier than originally designed, each 18 metres (59 ft) across. Following his death, the work continued under his assistant Jacopo Barozzi da Vignola with Giorgio Vasari appointed by Pope Pius V as a watchdog to make sure that Michelangelo's plans were carried out exactly. Despite Vignola's knowledge of Michelangelo's intentions, little happened in this period. In 1585 the energetic Pope Sixtus appointed Giacomo della Porta who was to be assisted by Domenico Fontana. The five year reign of Sixtus was to see the building advance at a great rate. Michelangelo left a few drawings, including an early drawing of the dome, and some drawings of details. There were also detailed engravings published in 1569 by Stefan du Pérac who claimed that they were the master's final solution. Michelangelo, like Sangallo before him, also left a large wooden model. Giacomo della Porta subsequently altered this model in several ways, in keeping with changes that he made to the design. Most of these changes were of a cosmetic nature, such as the adding of lion's masks over the swags on the drum in honour of Pope Sixtus and adding a circlet of finials around the spire at the top of the lantern, as proposed by Sangallo. The major change that was made to the model, either by della Porta, or Michelangelo himself before his death, was to raise the outer dome higher above the inner one. A drawing by Michelangelo indicates that his early intentions were towards an ovoid dome, rather than a hemispherical one. Stefan du Pérac's engraving shows a hemispherical dome, but this was perhaps an inaccuracy of the engraver. The profile of the wooden model is more ovoid than that of the engraving, but less so than the finished product. It has been suggested that Michelangelo on his death bed reverted to the more pointed shape. However Lees-Milne cites Giacomo della Porta as taking full responsibility for the change and as indicating to Pope Sixtus that Michelangelo was lacking in the scientific understanding of which he himself was capable. Helen Gardner suggests that Michelangelo made the change to the hemispherical dome of lower profile in order to establish a balance between the dynamic vertical elements of the encircling giant order of pilasters and a more static and reposeful dome. Gardner also comments "The sculpturing of architecture Michelangelo... here extends itself up from the ground through the attic stories and moves on into the drum and dome, the whole building being pulled together into a unity from base to summit." It is this sense of the building being sculptured, unified and "pulled together" by the encircling band of the deep cornice that led Eneide Mignacca to conclude that the ovoid profile, seen now in the end product, was an essential part of Michelangelo's first (and last) concept. The sculptor/architect has, figuratively speaking, taken all the previous designs in hand and compressed their contours as if the building were a lump of clay. The dome must appear to thrust upwards because of the apparent pressure created by flattening the building's angles and restraining its projections. If this explanation is the correct one, then the profile of the dome is not merely a structural solution, as perceived by Giacomo della Porta; it is part of the integrated design solution that is about visual tension and compression. In one sense, Michelangelo's dome may appear to look backward to the Gothic profile of Florence Cathedral and ignore the Classicism of the Renaissance, but on the other hand, perhaps more than any other building of the 16th century, it prefigures the architecture of the Baroque.